zram: Set initial disksize to some default value

Currently, we set initial disksize as 0, which forces user to write some
value to corresponding zram device's sysfs node, before the device can be
used. Now, we avoid this step by providing some default size initially.

To change the disksize, user must:
 - Reset disk.
	Ex: echo 1 > /sys/block/zram0/reset
(NOTE: disksize is set to the default value after reset)
 - Set new disksize.
	Ex: echo $((256*1024*1024)) > /sys/block/zram0/disksize
Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Reviewed-by: Jerome Marchand <jmarchan@redhat.com>
modified by faux123